A variety of physiological processes are electrically mediated and produce associated electrical signals. For example, the sinoatrial node in a human heart generates an electrical pulse that triggers the remainder of a heartbeat in a normally functioning heart. This pulse propagates through the heart's normal conduction pathways producing electrical signals that can be observed on the surface of a patient's body. Monitoring and analysis of such electrical signals have proved beneficial in evaluating the function of a patient's heart, including the detection of conditions associated with acute cardiac ischemia.
Monitoring of a patient's cardiac electrical activity is conventionally performed using a 12-lead electrocardiogram (ECG) system that includes a monitor and ten electrodes attached to a patient. A conventional 12-lead ECG system monitors the voltages sensed by the ten electrodes and generates twelve combinations of these voltages to produce the "leads" required by the 12-lead ECG system. Of the ten electrodes in a 12-lead ECG system, four are "limb" electrodes typically placed on or near each of a patient's four limbs, and six are "precordial" electrodes positioned on the patient's chest over the heart. As an electrical impulse propagates through the heart, the monitor repetitively measures the voltages sensed by the electrodes. Although the electrodes collectively monitor the same heartbeats, the electrodes sense different voltages due to their placement with respect to the patient's heart. A time sequence of monitored voltages is used to produce ECG lead data. An ECG monitor typically plots this data to provide graphical waveforms representing the heart's electrical activity for each lead being monitored.
An example of an ECG waveform is shown in FIG. 1. For purposes of analysis, an ECG waveform produced over a time interval corresponding to one cardiac cycle or "heartbeat" is divided into a number of waves. The portion of a waveform representing depolarization of the atrial muscle fibers is referred to as the "P" wave. Depolarization of the ventricular muscle fibers is collectively represented by the "Q," "R," and "S" waves of the waveform. The portion of the waveform representing repolarization of the ventricular muscle fibers is known as the "T" wave. Between heartbeats, an ECG waveform returns to an "isopotential" line. FIG. 1 also illustrates selected fiducial points labeled "q", "j", "t1", and "t2". Fiducial points define the boundaries of selected features and are used in measuring characteristics of an ECG waveform, such as the start and end of a heartbeat and the elevation of the ST portion of a heartbeat. The "q" point shown in FIG. 1 represents the start of the Q wave, the "j" point represents the end of the QRS complex, the "t1" point represents the start of the T wave, and the "t2" point represents the end of the T wave.
As noted, an analysis of a patient's ECG may assist in detecting acute cardiac ischemia in the patient. As a matter of background, acute cardiac ischemia is a condition that arises from chronic or sudden onset of deprivation of blood, and hence oxygen, to muscles of the heart. If an ischemic condition is severe or prolonged, it can result in irreversible death or damage to myocardial cells (i.e., an infarction). A chronic cardiac ischemic condition, angina, is typically caused by narrowing of the coronary arteries due to spasms of the wall muscles or partial blockage by plaques. A sudden cardiac ischemic condition may be caused by a clot blocking the passage of blood in the coronary arteries. Symptoms of a cardiac ischemic event may include chest pain and pain radiating through the extremities, but not all such events present these symptoms. Current medical intervention for severe acute ischemic events includes the administration of a class of drugs called thrombolytics that dissolve clots in the occluded coronary artery, and emergent PTCA, a medical procedure that opens the artery by inflating a balloon inside the clot to make a passage for circulation.
The amount of damage done to a heart by an ischemic event depends, in part, on the amount of time that lapses before treatment is provided. Therefore, ECG data should be evaluated as early as possible so that functional changes associated with cardiac ischemia can be detected and reported as early as possible. With early detection of acute cardiac ischemia, appropriate treatment can take place as early as possible and, thereby, maximize the preservation of myocardium. The American Heart Association recommends that a patient with suspected acute cardiac ischemia be evaluated by a physician using a 12-lead ECG within ten minutes of arrival at a hospital emergency department. Unfortunately, outside of a hospital, highly trained medical personnel are not always available to meet a patient's immediate needs. Quite often, a first responding caregiver to a patient in the field may not be competent in evaluating ECG waveforms to detect acute cardiac ischemic events. A need, therefore, exists for a device that not only obtains ECG data, but also quickly evaluates and automatically produces a preliminary diagnosis that an acute cardiac ischemic condition has been detected.
Traditionally, acute cardiac ischemic conditions are detected by a physician's visual evaluation of 12-lead ECG waveforms. A physician typically selects one or more leads in the 12-lead ECG and makes an initial assessment by comparing selected features of the patient's ECG waveforms to equivalent features of other persons' ECG waveforms that are representative of various abnormal conditions. A physician may also look at the patient's ECG waveforms over time and evaluate any changes in waveform shape. A number of waveform features have been identified as useful in diagnosing acute cardiac ischemic conditions. Customarily, a physician observes the extent to which the ST portion of a waveform exceeds the isopotential line (i.e., the ST elevation) and uses this information to determine if an acute ischemic event has occurred. Nevertheless, because of the subtleties involved in evaluating ECG waveforms, even highly trained individuals often fail to correctly diagnose an acute cardiac ischemic event when evaluating an ECG using traditional features alone. More subtle, globally distributed ECG features remain undetected. A deed, therefore, exists for more accurate ways of detecting and reporting acute cardiac ischemic events.
In recent years, there have been efforts to develop enhanced ECG waveform interpretation based on computer analysis. Conventional computer processes used for ECG waveform analysis are based on heuristics derived from the experience of expert physicians. Such processes implement rules that attempt to simulate an expert physician's reasoning but perform no better than the expert. In practice, many such processes perform more poorly than human expert evaluation.
Furthermore, when a conventional heuristic process is used, it is difficult to choose an optimal operating point for the device in terms of sensitivity (i.e., detecting true positives) and specificity (i.e., avoiding false positives). A device tuned to be more sensitive is typically less specific, while a device tuned to be more specific is typically less sensitive. A typical sensitivity/specificity tradeoff is illustrated by a receiver operating characteristics (ROC) curve, an example of which is shown in FIG. 15. Using a conventional heuristic process, it is difficult to make the sensitivity/specificity tradeoff explicit; thus, the selection of an operating point on the ROC curve is often made in a suboptimal, ad hoc manner. As such, there is a need for an apparatus and method that can provide better performance in terms of sensitivity and specificity and further provide for selecting a sensitivity/specificity tradeoff in a more systematic way.
The present invention provides a method and apparatus having such features, as well as addressing other shortcomings in the prior art.